Enhanced firing of locus coeruleus neurons and SK channel dysfunction are conserved in distinct models of prodromal Parkinson's disease.

Parkinson's disease (PD) is clinically defined by the presence of the cardinal motor symptoms, which are associated with a loss of dopaminergic nigrostriatal neurons in the substantia nigra pars compacta (SNpc). While SNpc neurons serve as the prototypical cell-type to study cellular vulnerability in PD, there is an unmet need to extent our efforts to other neurons at risk. The noradrenergic locus coeruleus (LC) represents one of the first brain structures affected in Parkinson's disease (PD) and plays not only a crucial role for the evolving non-motor symptomatology, but it is also believed to contribute to disease progression by efferent noradrenergic deficiency. Therefore, we sought to characterize the electrophysiological properties of LC neurons in two distinct PD models: (1) in an in vivo mouse model of focal α-synuclein overexpression; and (2) in an in vitro rotenone-induced PD model. Despite the fundamental differences of these two PD models, α-synuclein overexpression as well as rotenone exposure led to an accelerated autonomous pacemaker frequency of LC neurons, accompanied by severe alterations of the afterhyperpolarization amplitude. On the mechanistic side, we suggest that Ca2+-activated K+ (SK) channels are mediators of the increased LC neuronal excitability, as pharmacological activation of these channels is sufficient to prevent increased LC pacemaking and subsequent neuronal loss in the LC following in vitro rotenone exposure. These findings suggest a role of SK channels in PD by linking α-synuclein- and rotenone-induced changes in LC firing rate to SK channel dysfunction.

Neural correlates of vocal initiation in the VTA/SNc of juvenile male zebra finches.

Initiation and execution of complex learned vocalizations such as human speech and birdsong depend on multiple brain circuits. In songbirds, neurons in the motor cortices and basal ganglia circuitry exhibit preparatory activity before initiation of song, and that activity is thought to play an important role in successful song performance. However, it remains unknown where a start signal for song is represented in the brain and how such a signal would lead to appropriate vocal initiation. To test whether neurons in the midbrain ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) show activity related to song initiation, we carried out extracellular recordings of VTA/SNc single units in singing juvenile male zebra finches. We found that a subset of VTA/SNc units exhibit phasic activity precisely time-locked to the onset of the song bout, and that the activity occurred specifically at the beginning of song. These findings suggest that phasic activity in the VTA/SNc represents a start signal that triggers song vocalization.

The tectonigral pathway regulates appetitive locomotion in predatory hunting in mice.

Appetitive locomotion is essential for animals to approach rewards, such as food and prey. The neuronal circuitry controlling appetitive locomotion is unclear. In a goal-directed behavior-predatory hunting, we show an excitatory brain circuit from the superior colliculus (SC) to the substantia nigra pars compacta (SNc) to enhance appetitive locomotion in mice. This tectonigral pathway transmits locomotion-speed signals to dopamine neurons and triggers dopamine release in the dorsal striatum. Synaptic inactivation of this pathway impairs appetitive locomotion but not defensive locomotion. Conversely, activation of this pathway increases the speed and frequency of approach during predatory hunting, an effect that depends on the activities of SNc dopamine neurons. Together, these data reveal that the SC regulates locomotion-speed signals to SNc dopamine neurons to enhance appetitive locomotion in mice.

Dopamine Transporter Is a Master Regulator of Dopaminergic Neural Network Connectivity.

Dopaminergic neurons of the substantia nigra pars compacta (SNC) and ventral tegmental area (VTA) exhibit spontaneous firing activity. The dopaminergic neurons in these regions have been shown to exhibit differential sensitivity to neuronal loss and psychostimulants targeting dopamine transporter. However, it remains unclear whether these regional differences scale beyond individual neuronal activity to regional neuronal networks. Here, we used live-cell calcium imaging to show that network connectivity greatly differs between SNC and VTA regions with higher incidence of hub-like neurons in the VTA. Specifically, the frequency of hub-like neurons was significantly lower in SNC than in the adjacent VTA, consistent with the interpretation of a lower network resilience to SNC neuronal loss. We tested this hypothesis, in DAT-cre/loxP-GCaMP6f mice of either sex, when activity of an individual dopaminergic neuron is suppressed, through whole-cell patch clamp electrophysiology, in either SNC or VTA networks. Neuronal loss in the SNC increased network clustering, whereas the larger number of hub-neurons in the VTA overcompensated by decreasing network clustering in the VTA. We further show that network properties are regulatable via a dopamine transporter but not a D2 receptor dependent mechanism. Our results demonstrate novel regulatory mechanisms of functional network topology in dopaminergic brain regions.SIGNIFICANCE STATEMENT In this work, we begin to untangle the differences in complex network properties between the substantia nigra pars compacta (SNC) and VTA, that may underlie differential sensitivity between regions. The methods and analysis employed provide a springboard for investigations of network topology in multiple deep brain structures and disorders.

Precisely timed dopamine signals establish distinct kinematic representations of skilled movements.

Brain dopamine is critical for normal motor control, as evidenced by its importance in Parkinson Disease and related disorders. Current hypotheses are that dopamine influences motor control by 'invigorating' movements and regulating motor learning. Most evidence for these aspects of dopamine function comes from simple tasks (e.g. lever pressing). Therefore, the influence of dopamine on motor skills requiring multi-joint coordination is unknown. To determine the effects of precisely timed dopamine manipulations on the performance of a complex, finely coordinated dexterous skill, we optogenetically stimulated or inhibited midbrain dopamine neurons as rats performed a skilled reaching task. We found that reach kinematics and coordination between gross and fine movements progressively changed with repeated manipulations. However, once established, rats transitioned abruptly between aberrant and baseline reach kinematics in a dopamine-dependent manner. These results suggest that precisely timed dopamine signals have immediate and long-term influences on motor skill performance, distinct from simply 'invigorating' movement.

Stimulation of retrotrapezoid nucleus Phox2b-expressing neurons rescues breathing dysfunction in an experimental Parkinson's disease rat model.

Emerging evidence from multiple studies indicates that Parkinson's disease (PD) patients suffer from a spectrum of autonomic and respiratory motor deficiencies in addition to the classical motor symptoms attributed to substantia nigra degeneration of dopaminergic neurons. Animal models of PD show a decrease in the resting respiratory rate as well as a decrease in the number of Phox2b-expressing retrotrapezoid nucleus (RTN) neurons. The aim of this study was to determine the extent to which substantia nigra pars compact (SNc) degeneration induced RTN biomolecular changes and to identify the extent to which RTN pharmacological or optogenetic stimulations rescue respiratory function following PD-induction. SNc degeneration was achieved in adult male Wistar rats by bilateral striatal 6-hydroxydopamine injection. For proteomic analysis, laser capture microdissection and pressure catapulting were used to isolate the RTN for subsequent comparative proteomic analysis and Ingenuity Pathway Analysis (IPA). The respiratory parameters were evaluated by whole-body plethysmography and electromyographic analysis of respiratory muscles. The results confirmed reduction in the number of dopaminergic neurons of SNc and respiratory rate in the PD-animals. Our proteomic data suggested extensive RTN remodeling, and that pharmacological or optogenetic stimulations of the diseased RTN neurons promoted rescued the respiratory deficiency. Our data indicate that despite neuroanatomical and biomolecular RTN pathologies, that RTN-directed interventions can rescue respiratory control dysfunction.

Aging impairs perceptual decision-making in mice: integrating computational and neurobiological approaches.

Decision-making is one of the cognitive domains which has been under-investigated in animal models of cognitive aging along with its neurobiological correlates. This study investigated the latent variables of the decision process using the hierarchical drift-diffusion model (HDDM). Neurobiological correlates of these processes were examined via immunohistochemistry. Young (n = 11, 4 months old), adult (n = 10, 10 months old), and old (n = 10, 18 months old) mice were tested in a perceptual decision-making task (i.e. two-alternative forced-choice; 2AFC). Observed data showed that there was an age-dependent decrease in the accuracy rate of old mice while response times were comparable between age groups. HDDM results revealed that age-dependent accuracy difference was a result of a decrease in the quality of evidence integration during decision-making. Significant positive correlations observed between evidence integration rate and the number of tyrosine hydroxylase positive (TH+) neurons in the ventral tegmental area (VTA) and axon terminals in dorsomedial striatum (DMS) suggest that decrease in the quality of evidence integration in aging is related to decreased function of mesocortical and nigrostriatal dopamine.

Dopamine Evokes a Trace Amine Receptor-dependent Inward Current that is Regulated by AMP Kinase in Substantia Nigra Dopamine Neurons.

We reported recently that activators of AMP-activated protein kinase (AMPK) slow the rundown of current evoked by the D2 autoreceptor agonist quinpirole in rat substantia nigra compacta (SNC) dopamine neurons. The present study examined the effect of AMPK on current generated by dopamine, which unlike quinpirole, is a substrate for the dopamine transporter (DAT). Using whole-cell patch-clamp, we constructed current-voltage (I-V) plots while superfusing brain slices with dopamine (100 µM) for 25 min. Two minutes after starting superfusion, dopamine evoked a peak current with an average slope conductance of 0.97 nS and an estimated reversal potential (Erev) of -113 mV, which is near that expected for K+. But after 10 min of superfusion, dopamine-evoked currents had shifted to more depolarized values with a slope conductance of 0.64 nS and an Erev of -83 mV. This inward shift in current was completely blocked by the DAT inhibitor GBR12935. However, an AMPK blocking agent (dorsomorphin) permitted the emergence of inward current despite the continued presence of the DAT inhibitor. When D2 autoreceptors were blocked by sulpiride, I-V plots showed that dopamine evoked an inward current with an estimated slope conductance of 0.45 nS with an Erev of -57 mV. Moreover, this inward current was completely blocked by the trace amine-associated receptor 1 (TAAR1) antagonist EPPTB. These results suggest that dopamine activates a TAAR1-dependent non-selective cation current that is regulated by AMPK.

Subsaturation of the N-methyl-D-aspartate receptor glycine site allows the regulation of bursting activity in juvenile rat nigral dopamine neurons.

The activation of N-methyl-D-aspartate receptors (NMDARs) in substantia nigra pars compacta (SNc) dopamine (DA) cells is central to generate the bursting activity, a phasic signal linked to DA-related behaviours via the change in postsynaptic DA release. NMDARs are recruited during excitatory synaptic transmission by glutamate release, but the glycine site level of occupancy of these receptors during basal action potential-dependent activity is not known for SNc DA neurons. We explored NMDAR-dependent signals during exogenous applications of co-agonists in midbrain slices from juvenile rats. We found that both glycine and D-serine strengthened the NMDAR-dependent component of excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner. EPSCs were also increased by endogenous glycine via the blockade of the glycine transport. The glycine site of NMDARs contributing to synaptic transmission is therefore subsaturated. The behaviourally relevant burst firing was more sensitive to exogenous D-serine and endogenous glycine than to exogenous glycine. The mechanisms regulating the availability of the co-agonists exert consequently a critical influence on the excitability of DA neurons via NMDARs. The modulation of the phasic firing in DA neurons by ambient NMDAR co-agonists may be important for nigral information processing and downstream motor-related behaviour.

Blockade of calcium-permeable AMPA receptors in the lateral habenula produces increased antidepressant-like effects in unilateral 6-hydroxydopamine-lesioned rats compared to sham-lesioned rats.

At present, role of the lateral habenula (LHb) calcium-permeable AMPA receptors (CP-AMPARs) in depression is not understood, particularly in Parkinson's disease-related depression. Here we found that lesions of the substantia nigra pars compacta (SNc) in rats induced depressive-like behaviors, and intra-LHb injection of CP-AMPAR antagonist Naspm produced antidepressant-like effects in SNc sham-lesioned and SNc-lesioned rats, however, the doses inducing these effects in SNc-lesioned rats were lower than that of SNc sham-lesioned rats. Blockade of LHb CP-AMPARs decreased the firing rate of the neurons and increased release of dopamine and serotonin in the medial prefrontal cortex (mPFC) in both groups, but the duration of Naspm action on the firing rate and release of the transmitters were prolonged in SNc-lesioned rats. These changes in SNc-lesioned rats were involved in increased expression of ßCaMKII and p-GluR1-S831 in the LHb. Intra-LHb injection of Naspm inhibited dopaminergic neurons in the anterior ventral tegmental area and serotonergic neurons in the dorsal raphe nucleus and excited dopaminergic neurons in the posterior ventral tegmental area (pVTA) and serotonergic neurons in the median raphe nucleus (MRN), and lesioning the GABAergic rostromedial tegmental nucleus (RMTg) decreased the percentages of excited pVTA dopaminergic neurons and MRN serotonergic neurons. Our findings indicate that blockade of LHb CP-AMPARs produces antidepressant-like effects, which attribute to decreased firing activity of LHb neurons and increased levels of dopamine and serotonin in the mPFC, and provide further evidence that LHb CP-AMPARs regulate the firing activity of pVTA dopaminergic neurons and MRN serotonergic neurons indirectly via the RMTg.

Altered expression of dopaminergic cell fate regulating genes prior to manifestation of symptoms in a transgenic rat model of Huntington's disease.

Hyperactivity of the dopaminergic pathway is thought to contribute to clinical symptoms in the early stages of Huntington's disease (HD). It is suggested to be result of a reduced dopaminergic inhibition by degeneration of medium spiny neurons in the striatum. Previously, we have shown that the number of dopaminergic cells is increased in the dorsal raphe nucleus (DRN) of HD patients and transgenic HD (tgHD) rats during the manifestation phase of the disease; as well as in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) of tgHD rats. To address whether these changes are secondary to neurodegeneration or take place in the pre-manifest phase of the disease, we examined the expression of genes controlling neuronal cell fate and genes that define dopaminergic cell phenotype. In the SNc-VTA of tgHD rats, Msx1 was upregulated, which correlated with an altered expression of transcription factors Zbtb16 and Tcf12. Zbtb16 was upregulated in the DRN and it was the only gene that showed a correlated expression in the tgHD rats between SNc-VTA and DRN. Zbtb16 may be a candidate for regionally tuning its cell populations, resulting in the increase in dopaminergic cells observed in our previous studies. Here, we demonstrated an altered expression of genes related to dopaminergic cell fate regulation in the brainstem of 6 months-old tgHD rats. This suggests that changes in dopaminergic system in HD precede the manifestation of clinical symptoms, contradicting the theory that hyperdopaminergic status in HD is a consequence of neurodegeneration in the striatum.

Financial gain- and loss-related BOLD signals in the human ventral tegmental area and substantia nigra pars compacta.

Neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNC) play central roles in reward-related behaviours. Nonhuman animal studies suggest that these neurons also process aversive events. However, our understanding of how the human VTA and SNC responds to such events is limited and has been hindered by the technical challenge of using functional magnetic resonance imaging (fMRI) to investigate a small structure where the signal is particularly vulnerable to physiological noise. Here we show, using methods optimized specifically for the midbrain (including high-resolution imaging, a novel registration protocol, and physiological noise modelling), a BOLD (blood-oxygen-level dependent) signal to both financial gain and loss in the VTA and SNC, along with a response to nil outcomes that are better or worse than expected in the VTA. Taken together, these findings suggest that the human VTA and SNC are involved in the processing of both appetitive and aversive financial outcomes in humans.

Action potential and calcium dependence of tonic somatodendritic dopamine release in the Substantia Nigra pars compacta.

Despite the importance of somatodendritic dopamine (DA) release in the Substantia Nigra pars compacta (SNc), its mechanism remains poorly understood. Using a novel approach combining fast-scan controlled-adsorption voltammetry (FSCAV) and single-unit electrophysiology, we have investigated the mechanism of somatodendritic release by directly correlating basal (non-stimulated) extracellular DA concentration ([DA]out ), with pharmacologically-induced changes of firing of nigral dopaminergic neurons in rat brain slices. FSCAV measurements indicated that basal [DA]out in the SNc was 40.7 ± 2.0 nM (at 34 ± 0.5°C), which was enhanced by amphetamine, cocaine, and L-DOPA, and reduced by VMAT2 inhibitor, Ro4-1284. Complete inhibition of firing by TTX decreased basal [DA]out , but this reduction was smaller than the effect of D2 receptor agonist, quinpirole. Despite similar effects on neuronal firing, the larger decrease in [DA]out evoked by quinpirole was attributed to cell membrane hyperpolarization and greater reduction in cytosolic free Ca2+ ([Ca2+ ]in ). Decreasing extracellular Ca2+ also reduced basal [DA]out , despite increasing firing frequency. Furthermore, inhibiting L-type Ca2+ channels decreased basal [DA]out , although specific Cav 1.3 channel inhibition did not affect firing rate. Inhibition of sarcoplasmic/endoplasmic reticulum Ca2+ -ATPase (SERCA) also decreased [DA]out , demonstrating the importance of intracellular Ca2+ stores for somatodendritic release. Finally, in vivo FSCAV measurements showed that basal [DA]out in the SNc was 79.8 ± 10.9 nM in urethane-anesthetized rats, which was enhanced by amphetamine. Overall, our findings indicate that although tonic somatodendritic DA release is largely independent of action potentials, basal [DA]out is strongly regulated by voltage-dependent Ca2+ influx and release of intracellular Ca2+ . OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Local Field Potentials Reflect Dopaminergic and Non-Dopaminergic Activities within the Primate Midbrain.

Midbrain dopamine neurons are thought to play a crucial role in motivating behaviors toward desired goals. While the activity of dopamine single-units is known to adhere closely to the reward prediction error (RPE) signal hypothesized by learning theory, much less is known about the dynamic coordination of population-level neuronal activities in the midbrain. Local field potentials (LFPs) are thought to reflect the changes in membrane potential synchronized across a population of neurons nearby a recording electrode. These changes involve complex combinations of local spiking activity with synaptic processing that are difficult to interpret. Here we sampled LFPs from the substantia nigra pars compacta (SNc) of behaving monkeys to determine if local population-level synchrony encodes specific aspects of a reward/effort instrumental task and whether dopamine single-units participate in that signal. We found that reward-correlated information is encoded in a low-frequency signal (<32-Hz; delta and beta bands) that is synchronized across a neural population that includes dopamine neurons. Conversely, high-frequency power (>33-Hz; gamma band) was anticorrelated with predicted reward value and dopamine single-units were never phase-locked to those frequencies. This high-frequency signal may reflect inhibitory processes that were not otherwise observable. LFP encoding of movement-related parameters was negligible. Together, LFPs provide novel insights into the multidimensional processing of reward information subserved by dopaminergic and other components of the midbrain.

Brain sites mediating cyclosomatostatin-induced catalepsy in Wistar rats: A specific role for the nigrostriatal system and locus coeruleus.

A decrease in somatostatin activity is observed in the Parkinsonian brain. In recent experiments on rats, we simulated this abnormality by intracerebroventricular injections of a somatostatin antagonist, cyclosomatostatin. The treated animals displayed catalepsy, a state that resembles the extrapyramidal signs of Parkinson's disease. The neuroanatomical substrates mediating the catalepsy-inducing effect of cyclosomatostatin are unknown. To clarify this issue, we assessed here the action of cyclosomatostatin injected into the substantia nigra pars compacta (SNc), dorsal striatum (DS), locus coeruleus (LC), pedunculopontine tegmental nucleus (PPTg), and inferior colliculus (IC). The experiments were conducted with male Wistar rats of 270-290 g bw, catalepsy was evaluated by using the bar test. The injections into the PPTg and IC were without effect whereas the intra-SNc, intra-DS, and intra-LC administrations produced distinct cataleptic response. Thus, it was shown for the first time that the LC is a brain center capable of causing catalepsy. These data provide new insights into the neuroanatomical organization of the catalepsy-initiating mechanism and suggest the LC representing a potential target for therapeutic manipulations of extrapyramidal dysfunctions.

The rostromedial tegmental nucleus is essential for non-rapid eye movement sleep.

The rostromedial tegmental nucleus (RMTg), also called the GABAergic tail of the ventral tegmental area, projects to the midbrain dopaminergic system, dorsal raphe nucleus, locus coeruleus, and other regions. Whether the RMTg is involved in sleep-wake regulation is unknown. In the present study, pharmacogenetic activation of rat RMTg neurons promoted non-rapid eye movement (NREM) sleep with increased slow-wave activity (SWA). Conversely, rats after neurotoxic lesions of 8 or 16 days showed decreased NREM sleep with reduced SWA at lights on. The reduced SWA persisted at least 25 days after lesions. Similarly, pharmacological and pharmacogenetic inactivation of rat RMTg neurons decreased NREM sleep. Electrophysiological experiments combined with optogenetics showed a direct inhibitory connection between the terminals of RMTg neurons and midbrain dopaminergic neurons. The bidirectional effects of the RMTg on the sleep-wake cycle were mimicked by the modulation of ventral tegmental area (VTA)/substantia nigra compacta (SNc) dopaminergic neuronal activity using a pharmacogenetic approach. Furthermore, during the 2-hour recovery period following 6-hour sleep deprivation, the amount of NREM sleep in both the lesion and control rats was significantly increased compared with baseline levels; however, only the control rats showed a significant increase in SWA compared with baseline levels. Collectively, our findings reveal an essential role of the RMTg in the promotion of NREM sleep and homeostatic regulation.

Measurement of phase resetting curves using optogenetic barrage stimuli.

BACKGROUND: The phase resetting curve (PRC) is a primary measure of a rhythmically firing neuron's responses to synaptic input, quantifying the change in phase of the firing oscillation as a function of the input phase. PRCs provide information about whether neurons will synchronize due to synaptic coupling or shared input. However, PRC estimation has been limited to in vitro preparations where stable intracellular recordings can be obtained and background activity is minimal, and new methods are required for in vivo applications. NEW METHOD: We estimated PRCs using dense optogenetic stimuli and extracellular spike recording. Autonomously firing neurons in substantia nigra pars reticulata (SNr) of Thy1-channelrhodopsin 2 (ChR2) transgenic mice were stimulated with random barrages of light pulses, and PRCs were determined using multiple linear regression. RESULTS: The PRCs obtained were type-I, showing only phase advances in response to depolarizing input, and generally sloped upward from early to late phases. Secondary PRCs, indicating the effect on the subsequent ISI, showed phase delays primarily for stimuli arriving at late phases. Phase models constructed from the optogenetic PRCs accounted for a large fraction of the variance in ISI length and provided a good approximation of the spike-triggered average stimulus. COMPARISON WITH EXISTING METHODS: Compared to methods based on intracellular current injection, the new method sacrifices some temporal resolution. However, it should be much more widely applicable in vivo, because only extracellular recording and optogenetic stimulation are required. CONCLUSIONS: These results demonstrate PRC estimation using methods suitable for in vivo applications.

Differential Control of Dopaminergic Excitability and Locomotion by Cholinergic Inputs in Mouse Substantia Nigra.

Understanding how dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) govern movements requires a detailed knowledge of how different neurotransmitter systems modulate DA neuronal excitability. We report a heterogeneity of electrophysiological properties between medial and lateral SNc neurons modulated by cholinergic neurotransmission. Lateral DA neurons received mainly excitatory (nicotinic or glutamatergic) mediated cholinergic neurotransmission. Medial DA neurons received predominantly GABAergic currents mediated by presynaptic nicotinic receptors or biphasic GABAergic and nicotinic neurotransmission conveyed by GABA and ACh corelease, which inhibited DA neurons. To examine whether cholinergic signaling in the SNc controls mouse behavior, we used optogenetics in awake behaving mice and found that activation of cholinergic terminals in the medial SNc decreased locomotion, whereas activation in the lateral SNc increased locomotion. Our findings provide novel insights on how cholinergic inputs to subregions of the SNc regulate the excitability of DA neurons differentially, resulting in different patterns of motor behavior.

Cholinergic Mesopontine Signals Govern Locomotion and Reward through Dissociable Midbrain Pathways.

The mesopontine tegmentum, including the pedunculopontine and laterodorsal tegmental nuclei (PPN and LDT), provides major cholinergic inputs to midbrain and regulates locomotion and reward. To delineate the underlying projection-specific circuit mechanisms, we employed optogenetics to control mesopontine cholinergic neurons at somata and at divergent projections within distinct midbrain areas. Bidirectional manipulation of PPN cholinergic cell bodies exerted opposing effects on locomotor behavior and reinforcement learning. These motor and reward effects were separable via limiting photostimulation to PPN cholinergic terminals in the ventral substantia nigra pars compacta (vSNc) or to the ventral tegmental area (VTA), respectively. LDT cholinergic neurons also form connections with vSNc and VTA neurons; however, although photo-excitation of LDT cholinergic terminals in the VTA caused positive reinforcement, LDT-to-vSNc modulation did not alter locomotion or reward. Therefore, the selective targeting of projection-specific mesopontine cholinergic pathways may offer increased benefit in treating movement and addiction disorders.

Nigrostriatal Dopamine Acting on Globus Pallidus Regulates Sleep.

Lesions of the globus pallidus externa (GPe) produce a profound sleep loss (∼45%) in rats, suggesting that GPe neurons promote sleep. As GPe neuronal activity is enhanced by dopamine (DA) from the substantia nigra pars compacta (SNc), we hypothesized that SNc DA via the GPe promotes sleep. To test this hypothesis, we selectively destroyed the DA afferents to the caudoputamen (CPu) using 6-hydroxydopamine and examined changes in sleep-wake profiles in rats. Rats with 80-90% loss of SNc neurons displayed a significant 33.7% increase in wakefulness (or sleep reduction). This increase significantly correlated with the extent of SNc DA neuron loss. Furthermore, these animals exhibited sleep-wake fragmentation and reduced diurnal variability of sleep. We then optogenetic-stimulated SNc DA terminals in the CPu and found that 20-Hz stimulation from 9 to 10 PM increased total sleep by 69% with high electroencephalograph (EEG) delta power. We finally directly optogenetic-stimulated GPe neurons and found that 20-Hz stimulation of the GPe from 9 to 10 PM increased total sleep by 66% and significantly increased EEG delta power. These findings elucidate a novel circuit for DA control of sleep and the mechanisms of abnormal sleep in BG disorders such as Parkinson's disease and Huntington's disease.